This has been the problem and trade off since the inception of the z-buffer :)
It will get a little better with when get OpenGL 3.0 out in the wild as that has provision for a 32 bit Zbuffer which will extend the range 32 bit is still not great Gordon __________________________________________________________ Gordon Tomlinson Product Manager 3D Email : gtomlinson @ overwatch.textron.com __________________________________________________________ "Self defence is not a function of learning tricks but is a function of how quickly and intensely one can arouse one's instinct for survival" - Master Tambo Tetsura -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Chris Denham Sent: Wednesday, October 08, 2008 8:00 AM To: [email protected] Subject: Re: [osg-users] Near and far calculation and depth maps (and aquestion about WoWvx) Hmm, thinking about the implications of Wojtek's message in more detail leads me to think that the only way to get this right is to make sure the near plane doesn't get too close to the camera. In my simple test, a scene with small cubes inside a large cube, once my camera moves inside the large cube, the calculated near plane has a negative z value, which is then adjusted (by clampProjectionMatrix) to (nearFarRatio*zfar) because this is greater than calculated znear. Effectively, I think this means that once inside the large cube, the near plane is locked incredibly close to the camera, thus depth values for other things within the scene get pushed way back. My suggested change in my first message on this thread helps with this problem, but I seem to get the same effect whenever any object gets very close to the camera. I can workaround the problem by setting CullSettings::_nearFarRatio (default=0.0005) to a much higher value e.g. (0.1), but this of course has the unpleasant effect of clipping objects close to the camera. Oh damn it, I sense that I am treading on a very old computer graphics problem that human vision solves so well by being able to ignore near things that are not of interest or insignificant. Chris. ----- Original Message ----- From: "Chris Denham" <[EMAIL PROTECTED]> To: <[email protected]> Cc: <[EMAIL PROTECTED]> Sent: Wednesday, October 08, 2008 10:48 AM Subject: Re: [osg-users] Near and far calculation and depth maps (and a questionabout WoWvx) > Thanks Wojtek, > That's very useful information. I must admit, the non linear > relationship between depth map values and view space z coordinates was > something that escaped me. I had assumed the depth value spread was > linear between near and far planes. > > So that leaves me with a question about the wowvx disparity function. > How do I control its parameters so that it reliably generates a 'good' > greyscale rendering of the depthmap? By 'good' I mean one that has > enough contrast to allow the WoWvx display to show autostereoscopic effects. > > The fragment shader code for the wowvx disparity function used in OSG > look > like: > float disparity(float Z) > { > return > (wow_disparity_M*(1.0-(wow_disparity_vz/(Z-wow_disparity_Zd+wow_disparit y_vz))) > + wow_disparity_C) / 255.0; > } > > Where: > return value: The greyscale value to render. > Z: The depth value from depth map texture. > Default uniforms for WoWvx 20inch display wow_disparity_Zd = > 0.459813f; wow_disparity_vz = 6.180772f; wow_disparity_M = -1586.34f; > wow_disparity_C = 127.5f; > > If the depth map values from OSG are in general always optimal enough > to render the scene properly, then it seems that I need to dynamically > adjust (once per frame) the WowVX disparity constants to prevent the > autostereoscopic effect from fading in and out or disappearing as I > move the camera through a scene. > > If that's true, I have no idea how to proceed other than perhaps a > rather kludgy idea of generating a histogram of depth values and > somehow adjusting the disparity constants based on ranges of the > histogram that have 'detail'. That's ugly and unreliable! > > Having said that, I have actually looked at the histograms of the > depthmap values, and it did appear as though something might not be > quite right about the depth map because as I move through the scene, > the values seem occasionally get all bunched up at the far end (close > to 1.0), and these are the cases when I get problems. > > Help! > > Chris. > > >> Date: Tue, 7 Oct 2008 11:11:46 +0200 >> From: "Wojciech Lewandowski" <[EMAIL PROTECTED]> >> Subject: Re: [osg-users] Near and far calculation and depth maps (and >> a questionabout WoWvx) >> To: "OpenSceneGraph Users" <[email protected]> >> Message-ID: <[EMAIL PROTECTED]> >> Content-Type: text/plain; format=flowed; charset="ISO-8859-1"; >> reply-type=response >> >> Chris, >> >> Please read following page, until final conclusion: >> http://www.sjbaker.org/steve/omniv/love_your_z_buffer.html >> >> So if you have gone to the conclusion you should now understand why >> computeNearFar is added to OSG. It simply allows to better utilize >> depth buffer range. Compute NearFar using primitives is simply more >> precise than using bounding volumes so this may explain why you got >> better results with COMPUTE_NEAR_FAR_USING_PRIMITIVES. >> >> You should also know that depth buffer spread does not linearly >> correspond to view space z coordinate. Thats why when displayed as >> grayscale some objects may be hard to discern (especially if 24 bits >> are clamped to 8 bits). Depth buffer values are spread non linearly >> so when put into monochrome image they often end up with so similar >> colors that image looks completely flat (either white or black). You >> may want to add special shader to pseudo color depth buffer like in >> topographical maps to actually notice variety of depth values. >> >> I hope this helps a little ;-). >> >> Cheers, >> Wojtek >> >> >> >>>I have a few questions about the way OSG handles depth maps, and how >>>the range of depth values relates to the near and far calculation >>>done by OSG. >>> Firstly, just to confirm my understanding. I have assumed that the >>>purpose of OSG calculating near&far values is so that depth map >>>values will range between 0.0 and 1.0 for pixels rendered between >>>the near and far range. Oh.. and for clipping too!? Please shoot me >>>now if that's not correct. ;-) >>> >>> As a test case, I constructed a very simple scene comprised of a >>> large cuboid with a number of smaller cuboids within it. >>> When I move the camera inside the large cuboid, I had expected the >>> depth map rendered by an RTT camera to show depth values of all the >>> smaller cuboids in the scene, but it doesn't, it just seems to >>> contain a solid black texture. (you can easily display an inverted >>> representation of the depth map of a scene using the osgViewer >>> option '--wowvx-20') >>> >>> Now, the odd thing is that if I change the nearfar calculation mode >>> of the RTT camera to: >>> camera->setComputeNearFarMode(osg::CullSettings::ComputeNearFarMode: >>> camera->:COMPUTE_NEAR_FAR_USING_PRIMITIVES) >>> instead of the default, which is >>> COMPUTE_NEAR_FAR_USING_BOUNDING_VOLUMES >>> ,the smaller cubes now appear as I would have expected in the depthmap. >>> >>> I have been looking for an explantion for this difference in the OSG >>> code >>> in: >>> bool CullVisitor::updateCalculatedNearFar(const osg::Matrix& >>> matrix,const osg::Drawable& drawable, bool isBillboard) It seemed to >>> me that there may be an issue with the code: >>> ---snip--- >>> //if (d_near<0.0) osg::notify(osg::WARN)<<" 3) set near with >>> d_near="<<d_near<< std::endl; _computed_znear = d_near; >>> ---snip--- >>> which allows negative values of 'd_near' to be assigned to >>> '_computed_znear' and hence the near clipping plane to be behind the >>> camera. In fact, the code a few lines above this snippet does ignore >>> values of d_near < 0.0, so I wondered if this snippet should have >>> the same logic. In fact, if I change the snippet above (by removing >>> the comment and putting in an else), ---new snip--- if (d_near<0.0) >>> osg::notify(osg::WARN)<<" 3) set near with d_near="<<d_near<< >>> std::endl; else _computed_znear = d_near; ---new snip--- the depth >>> map seems correct for my test example, yippie! Let me know if I need >>> to put this change on the submissions list. >>> >>> Ah ha.... nope.. I haven't finished with your attention yet ;-) >>> >>> What follows is a different issue, only loosely related to the above: >>> I can see that I should expect to get a kind of 'popping' effect in >>> a depth map range due to culling of objects as I move through as >>> scene. It seemed to me that getting 'smooth' transitions in near/far >>> calculations would be 'at least in concept' a difficult/impossible problem to solve. >>> You are probably wondering why I care about this, when a even large >>> jitters in depth map range does not seem to affect the rendered >>> scene much. The answer is, because the depth map is used as part of >>> the input to a WoWvx autostereoscopic display. The problem is that >>> to transfer the depthmap data to the stereoscopic display, the >>> depthmap gets rendered as a greyscale image, presumably losing a lot >>> of precision in converting to int >>> 0-255 range greyscale color value. The effect I get as I move >>> through a scene is that the contrast in the depthmap image seems to >>> fade in and out as I move the camera past objects in the scene, and >>> this affects the 3D effect you get from the WoWvx. >>> I can see that the WoWvx implementation in OSG does include some >>> disparity calculation shader code that allows adjustment of the >>> conversion from depthmap value to greyscale, but it is difficult to >>> see how to calculate the parameters 'on the fly' as you move through >>> a scene. Does anyone have any suggestions or advice on how >>> dynamically keep more contrast in greyscale rendering of a depthmap? >>> >>> Chris. > _______________________________________________ osg-users mailing list [email protected] http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.or g _______________________________________________ osg-users mailing list [email protected] http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
